Re: [tree-ssa] Fix dominator bug

[Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>]

Hello,

> > > > Which reminds me, have you seen any compile-time
> > > > regressions
> > > > due to this change, Steven?
> > > 
> > > Only dead code removal is transformation I can think of that would work
> > > with this code without any other dominance tree manipulation.
> > > But dead basic blocks can't be in the dminator tree anyway (we abort
> > > while building the datastructure)
> > 
> > We do not (we have no way of knowing about them, once we throw the basic
> > block away).
> 
> I meant that we abort when we attempt to build dominator tree for CFG
> with unreachable basic blocks.
> I would agree that something should happen when basic block is removed
> and it is still in the dominator tree, just I would preffer it to be
> abort instead of silent removal of the block from the tree, since any
> transformation that makes blocks unreachable has to either invalidate
> the dominance or update it in more complex way by hand, so the abort
> would be reminder to do it instead of keeping tree silently
> inconsistent.

There is a verify_dominators call in tree_verify_flow_info, which would
inform you about this.  The basic block needs to be removed from the
dominance tree in any case, and doing it separately whenever you remove a
block seems clumsy.

> However since we do check this in verify_flow_info later anyway, I don't
> think this is critical, so I would be happy with the silent removal too.
> (do we notice such a forgotten blocks in verify_flow_info?)

No. I will add the check.

Zdenek